R. A. McGill

Nerve agent detection using networks of single-walled carbon nanotubes

We report the use of carbon nanotubes as a sensor for chemical nerve agents. Thin-film transistors constructed from random networks of single-walled carbon nanotubes were used to detect dimethyl methylphosphonate (DMMP), a simulant for the nerve agent sarin. These sensors are reversible and capable of detecting DMMP at sub-ppb concentration levels, and they are intrinsically selective against interferent signals from hydrocarbon vapors and humidity. We provide additional chemical specificity by the use of filters coated with chemoselective polymer films. These results indicate that the electronic detection of sub-ppb concentrations of nerve agents and potentially other chemical warfare agents is possible with simple-to-fabricate carbon nanotube devices.

Growth of organic thin films by the matrix assisted pulsed laser evaporation (MAPLE) technique

Abstract A novel variation of conventional pulsed laser evaporation, known as matrix assisted pulsed laser evaporation, or MAPLE, has been successfully used to deposit highly uniform thin films of a variety of organic materials including a number of polymers. The MAPLE technique is carried out in a vacuum chamber and involves directing a pulsed laser beam (λ=193 or 248 nm; fluence=0.01 to 0.5 J/cm2) onto a frozen target (100–200 K) consisting of a solute polymeric or organic compound dissolved in a solvent matrix. The laser beam evaporates the surface layers of the target, with both solvent and solute molecules being released into the chamber. The volatile solvent is pumped away, whereas the polymer/organic molecules coat the substrate. Thin uniform films ( nm) of various materials, such as functionalized polysiloxanes and carbohydrates, have been deposited on Si(111) and NaCl substrates. The films prepared using this method have been examined by optical microscopy, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy and electrospray mass spectrometry. Careful control of the processing conditions allowed the complex polymer/organic molecules to be transferred to the substrate as uniform films without any significant chemical modification. Using MAPLE, large or small regions within a substrate can be discretely coated with submonolayer thickness control. The use of MAPLE films for chemical sensor applications has been investigated and the potential of this technique for producing high quality thin films of other organic compounds will be discussed.

The deposition, structure, pattern deposition, and activity of biomaterial thin-films by matrix-assisted pulsed-laser evaporation (MAPLE) and MAPLE direct write

Two techniques, Matrix-Assisted Pulsed-Laser Evaporation (MAPLE) and MAPLE Direct Write (MDW) were developed to deposit biomaterial thin-films. MAPLE involves dissolving or suspending the biomaterial in a volatile solvent, freezing the mixture to create a solid target, and using a low fluence pulsed laser to evaporate the target for deposition of the solute inside a vacuum system. Using simple shadow masks, i.e. lines, dots and arrays, pattern features with length scales as small as 20 μm can be deposited using multiple materials on different types of substrates. MDW uses pulsed laser to directly transfer material from a ribbon to a substrate. Patterns with a spatial resolution of ∼10 μm can be written directly. Biomaterials ranging from polyethylene glycol to eukaryotic cells, i.e. Chinese hamster ovaries, were deposited with no measurable damage to their structures or genotype. Deposits of immobilized horseradish peroxidase, an enzyme, in the form of a polymer composite with a protective coating, i.e. polyurethane, retained their enzymatic functions. A dopamine electrochemical sensor was fabricated by MDW using a natural tissues/graphite composite. These examples and the unique features of MAPLE and MDW for biosensor fabrication have been discussed.

Laser transfer of biomaterials: Matrix-assisted pulsed laser evaporation (MAPLE) and MAPLE Direct Write

Two techniques for transferring biomaterial using a pulsed laser beam were developed: matrix-assisted pulsed laser evaporation (MAPLE) and MAPLE direct write (MDW). MAPLE is a large-area vacuum based technique suitable for coatings, i.e., antibiofouling, and MDW is a localized deposition technique capable of fast prototyping of devices, i.e., protein or tissue arrays. Both techniques have demonstrated the capability of transferring large (mol wt>100 kDa) molecules in different forms, e.g., liquid and gel, and preserving their functions. They can deposit patterned films with spatial accuracy and resolution of tens of μm and layering on a variety of substrate materials and geometries. MDW can dispense volumes less than 100 pl, transfer solid tissues, fabricate a complete device, and is computed aided design/computer aided manufacturing compatible. They are noncontact techniques and can be integrated with other sterile processes. These attributes are substantiated by films and arrays of biomaterials, e.g., polymers, enzymes, proteins, eucaryotic cells, and tissue, and a dopamine sensor. These examples, the instrumentation, basic mechanisms, a comparison with other techniques, and future developments are discussed.

New approach to laser direct writing active and passive mesoscopic circuit elements

We have combined some of the major positive advantages of laser-induced forward transfer (LIFT) and matrix-assisted pulsed laser evaporation (MAPLE), to produce a novel excimer laser driven direct writing technique which has demonstrated the deposition in air and at room temperature and with sub-10 μm resolution of active and passive prototype circuit elements on planar and nonplanar substrates. We have termed this technique MAPLE DW (matrix-assisted pulsed laser evaporation direct write) and present its historical evolution from pulsed laser deposition. This paper describes the simplistic approach to carry out MAPLE DW, gives experimental conditions, and physical characterization results for the deposition of NiCr thin film resistors, Au conducting lines, and multilayer depositions of Au conductors and BaTiO3 dielectrics to produce prototype capacitors. In general, the electrical properties of the materials deposited (conductivity, dielectric constant, and loss tangent) are comparable or superior to those produced by other commonly used industrial processes such as screen printing. The mechanism of the MAPLE DW process, especially the novel aspects making it a powerful approach for direct writing all classes of materials (metals, oxide ceramics, polymers and composites), is also described.

The effect of the matrix on film properties in matrix-assisted pulsed laser evaporation

Thin films of polyethylene glycol of average molecular weight 1400 amu have been deposited by matrix-assisted pulsed laser evaporation (MAPLE). The deposition was carried out in vacuum (∼10−6 Torr) with an ArF (λ=193 nm) laser at a fluence of 220–230 mJ/cm2. Films were deposited on NaCl plates and glass microscope slides. Both deionized water (H2O) and chloroform (CHCl3) were used as matrices. The physiochemical properties of the films are compared via Fourier transform infrared spectroscopy, and electrospray ionization mass spectrometry. The results show that the matrix used during MAPLE can greatly affect the chemical structure and molecular weight distribution of the deposited film. The infrared absorption spectrum shows evidence for C–Cl bond formation when CHCl3 is used as a matrix, while there is little evidence in the IR data for photochemical modification when H2O is used as a matrix. Time-of-flight analysis was performed using a quadrupole mass spectrometer to monitor evaporation of a frozen CHCl...

Stand-off detection of trace explosives via resonant infrared photothermal imaging

We describe a technique for rapid stand-off detection of trace explosives and other analytes of interest. An infrared (IR) laser is directed to a surface of interest, which is viewed using a thermal imager. Resonant absorption by the analyte at specific IR wavelengths selectively heats the analyte, providing a thermal contrast with the substrate. The concept is demonstrated using trinitrotoluene and cyclotrimethylenetrinitramine on transparent, absorbing, and reflecting substrates. Trace explosives have been detected from particles as small as 10 μm.

Direct writing of electronic and sensor materials using a laser transfer technique

We present a laser-based direct write technique termed matrix-assisted pulsed-laserevaporation direct write (MAPLE DW). This technique utilizes a laser transparentfused silica disc coated on one side with a composite matrix consisting of the materialto be deposited mixed with a laser absorbing polymer. Absorption of laser radiationresults in the decomposition of the polymer, which aids in transferring the solute to anacceptor substrate placed parallel to the matrix surface. Using MAPLE DW, complexpatterns consisting of metal powders, ceramic powders, and polymer composites weretransferred onto the surfaces of various types of substrates with <10 micron resolutionat room temperature and at atmospheric pressure without the use of masks.Current trends for developing advanced electronic andsensor systems place great emphasis in achieving per-formance levels generally associated with integratedcircuits. This requires further miniaturization, while en-hancing the functionality and reliability of existing sys-tems. New strategies are needed in order to eliminate thelong lead times required for the fabrication of prototypesand evaluation of new materials and designs. The use ofrapid prototyping techniques such as direct write, whichdo not need photolithographic processing, provide a so-lution to the above requirements. Direct write technolo-gies do not compete with photolithography for size andscale but rather add a complementary tool for specificapplications requiring rapid turnaround and/or patterniteration, conformal patterning, or modeling difficult cir-cuits. Examples of direct write technologies for fabricat-ing or modifying metallic interconnects and/or otherelectronic passive elements include ink jet printing,

Laser processing of polymer thin films for chemical sensor applications

Contemporary and next-generation commercial and defense-related platforms offer countless applications for thin-film polymer coatings, including the areas of microelectronics, optoelectronics, and miniature chemical and biological sensors. In many cases, the compositional and structural complexity, and the anisotropy of the material properties preclude the processing of many of these polymers by conventional physical or chemical vapor deposition methods. The Naval Research Laboratory has developed several advanced laser-based processing techniques for depositing polymer thin films for a variety of structures and devices. The two techniques detailed in this work, matrix-assisted pulsed laser evaporation (MAPLE) and MAPLE direct-write (MAPLE DW), are based on the concept of laser absorption by a matrix solution consisting of a solvent and the desired polymer. MAPLE is a physical vapor deposition process that takes place inside a vacuum chamber, while MAPLE DW is a laser forward-transfer process that is carried out under atmospheric conditions. Both processes have been successfully used in the fabrication of thin films and structures of a range of organic materials and systems. Examples of their use in the fabrication of two types of chemical sensors, together with a comparison of the performance of these laser-processed sensors and that of similar sensors made by traditional techniques are provided.

Resonant infrared pulsed-laser deposition of polymer films using a free-electron laser

Thin films of polyethylene glycol (MW 1500) have been prepared by pulsed-laser deposition (PLD) using both a tunable infrared (λ=2.9 μm, 3.4 μm) and an ultraviolet laser (λ=193 nm). A comparison of the physicochemical properties of the films by means of Fourier transform infrared spectroscopy, electrospray ionization mass spectrometry, and matrix-assisted laser desorption and ionization shows that when the IR laser is tuned to a resonant absorption in the polymer, the IR PLD thin films are identical to the starting material, whereas the UV PLD show significant structural modification. These results are important for several biomedical applications of organic and polymeric thin films.